Generally, an increase in a tire temperature of a pneumatic tire is considered to be undesirable in terms of durability because such temperature increase promotes a time-dependent change, which is a change in the physical properties of a material, or contributes to damages in a tread part and the like in high-speed driving. A reduction in the tire temperature for durability improvement is now an important task to be accomplished especially for tires used under heavy load, including off-the-road radial tires (ORR), truck-and-bus radial tires (TBR), and run-flat tires being driven while punctured (driven under an internal pressure of 0 kPa).
For example, when a run-flat tire having crescent-shaped sidewall reinforcing layers is driven while punctured, deformation in the tire radial direction concentrates on the sidewall reinforcing layers. Consequently, the temperature in the sidewall reinforcing layers rises so high as to affect durability considerably.
A technique has been disclosed for reducing the tire temperature in such a pneumatic tire (Japanese Patent Application Publication No. 2006-76431). In the technique, a reinforcing member is provided to mitigate or suppress deformation of components of the pneumatic tire (such as, particularly, a carcass layer at a part located at a sidewall part and a bead part).
However, the conventional pneumatic tire described above has a problem of degrading general driving performance, such as handling stability and ride comfort. This is because provision of the reinforcing member leads to an increase in the tire weight and to an occurrence of new unintended breakage such as separation at the reinforcing member. Particularly, in the run-flat tire, there is a concern that the reinforcing member increases vertical springing (elasticity in the tire vertical direction) of the tire driven at a normal internal pressure, resulting in degradation in general driving performance. A measure not impairing the general driving performance has therefore been sought.
The present invention has been made in consideration of such circumstances, and has an objective of providing a pneumatic tire capable of reducing the tire temperature efficiently while maintaining general driving performance.